1. Field of the Invention
This invention relates to integrated circuit manufacturing techniques, and more particularly to a technique using a self-aligned barrier of refractory material to prevent contact metal electromigration or "spiking", to provide good contact adherence, and to provide a junction oxidation mask.
2. Related Art
In manufacturing integrated circuits, a typical problem in using aluminum to form contacts to a semiconductor surface (such as silicon) is that the semiconductor material may dissolve into the aluminum. As the semiconductor material migrates into the aluminum, voids may be created in the semiconductor. The voids may penetrate the junction underneath the semiconductor material, creating excessive junction leakage current known as "spiking". Therefore, it would be desirable to develop a processing technique whereby such spiking is prevented. In the past, silicon doping was used in the metallic interconnect material, with the intention of preventing spiking. However, such silicon frequently precipitated into large nodules which often caused broken metal lines or unreliable contacts.
Some attempt has been made to use other metals such as molybdenum or tungsten as a contact material rather than aluminum. While such materials may not exhibit spiking problems, they do have problems in adequately adhering to many semiconductor materials.
The inventive solution to the problem of spiking is to deposit a metallic barrier to the aluminum. The barrier ideally should have a low contact resistance, be able to withstand thermal cycling up to temperatures of about 600.degree. C., have a low level of contaminants, and be easy to pattern with fine geometry lines.
The present invention accomplishes this goal by using a refractory metallic material, which, in the preferred embodiment, is titanium nitride (TiN). The TiN is used as a self-aligned deposition over silicon contact holes using a dual-level masking scheme. The TiN acts as a barrier metal preventing spiking of an overcoat of aluminum contact metal through the silicon junction. A deposition of TiN may also be used when other metal compositions (such as molybdenum) are used instead of aluminum, as they may more readily adhere to TiN.
Another problem associated with contact holes to a silicon junction is that it is at times difficult to insure that a metal interconnect layer makes adequate contact through an insulating oxide, due to the fact that holes that are typically etched through such an oxide are typically sharp edged, and can cause mechanical stress fractures in the overlying metal. To prevent such stress fractures, after contact holes are etched through an interlevel oxide, the semiconductor wafer is subjected to a temperature treatment designed to flow the interlevel oxide and provide smoother surface features. However, this process often causes a thin layer of silicon oxide, an insulator, to cover the surface of a silicon junction. The TiN acts as a barrier metal providing an oxidation mask that prevents oxidation of the contact holes during the processing step of contact etch flow.
Therefore, it is desirable to provide a means not only for preventing spiking of a contact metal through a semiconductor junction, or to promote adequate adherence of a contact metal, but also to prevent oxidation from occurring at the junction site when flow processing is used.